1. Field of the Invention
The present invention is directed to a vapor compression heat pump cycle which permits adjustment of the heat pump capacity over a wide range independently from outdoor conditions. More particularly, the present invention is directed to a method of transferring heat by a vapor compression heat pump cycle and an apparatus therefor, whereby a low pressure ratio for the compressor can be maintained.
2. Description of the Prior Art
German Pat. No. 84,084 discloses a method of refrigeration wherein a supersaturated ammonia solution is passed into an evaporator chamber, which is held at a lower pressure (higher than atmospheric), and a portion of the ammonia evaporates to produce a cooling effect. The so-formed gaseous ammonia is transferred with the weaker, but still supersaturated, ammonia solution to a chamber maintained at a higher pressure where the ammonia is reabsorbed, while being cooled. The reconstituted supersaturated ammonia solution is then once again fed into the evaporator.
French Pat. No. 537,438 discloses a refrigeration technique wherein a normally gaseous material (e.g., ammonia) is evaporated from a solution (e.g., an aqueous solution) in contact with coils containing a circulating fluid to cool the circulating fluid, in a desorber. The gaseous material is then pumped to an adsorber where it is contacted with the depleted aqueous solution, under pressure, so as to be readsorbed to reconstitute the aqueous solution. The adsorber is cooled by a cooling coil with a flow of cooling water therethrough. The reconstituted solution is further cooled by indirect countercurrent heat exchange with the depleted aqueous solution exiting the desorber before entering the desorber.
German Pat. No. 386863 discloses a heat transfer system wherein heat is transferred from a lower temperature body to a higher temperature body by use of two combined refrigeration cycles wherein one of the cycles is of the compression type and the other cycle is of the absorption type. For instance, the heat of flowing water at a temperature of 15.degree.-29.degree. C. is used to ultimately generate steam at two atmospheres pressure. In particular, a compressor compresses ammonia gas from 6 to 30 atm. (which would correspond to a condensation temperature of 66.degree. C.). The compressed ammonia is delivered into a first tank which contains an aqueous ammonia solution with 40% ammonia. This solution absorbs the compressed ammonia at a temperature of 130.degree. C. The heat released at this temperature is given off to a water tank which generates the steam at two atmospheres pressure. In order to replace the now-enriched solution in the first tank by a weaker solution, a second tank, from which the compressor obtains the ammonia gas is likewise filled with a 40 % ammonia solution. From this solution is formed, under a pressure of 6 atmospheres, the ammonia gas at a temperature of 60.degree. C. The required heat of evaporation of the ammonia is supplied by the second cyclic process. In this second cyclic process, the flowing water, which is available at a temperature of 15.degree. to 20.degree. C. gives off its heat to a pipe coil in which liquid ammonia evaporates at 6 atmospheres and 9.degree. C. The ammonia is compressed to 30 atmospheres pressure and forced through a pipe coil within the second tank. The ammonia condenses at 66.degree. C. and suffices to keep the second tank at 60.degree. C. The condensed liquid ammonia is returned to the pipe coil in contact with the flowing water through a steam trap.
German Pat. No. 953,378 discloses a heat pump system wherein the temperature difference between ground water and low outside temperature is utilized to open up a considerable energy source. In particular, heat of a medium temperature is brought from a heat reservoir (ground water, river water, waste heat) to a higher temperature and the energy expenditure for moving the heat is covered at least partly by utilizing the temperature gradient between the temperature of a colder medium (e.g., outside air) and the temperature of the heat reservoir by means of a counterflow absorption machine. In its simplest form, the heat pump system comprises a closed absorption system (e.g., aqueous ammonia) linked to a closed compression system (e.g., freon as working medium). The condenser of the absorption system is cooled by the evaporator of the compression system. The condenser of the compression system is cooled by outside air. The evaporator of the absorption system is heated by ground water. The heat from the resorber is used to supply heat to a dwelling place. The heat for the de-aerator is supplied by ground water.
More particularly, ammonia vapor, under pressure, is fed from the evaporator into the resorber, where it is absorbed by a lean solution to form a rich solution and gives up heat of absorption at a higher temperature. The so-formed rich solution is expanded into the de-aerator, which is heated by ground water, to regenerate a lean solution and ammonia vapor. The lean solution is pumped back to the resorber. The vapor is fed to the condenser where it is cooled by the evaporator of the closed compression system to form liquefied ammonia and the liquefied ammonia is fed into the evaporator to regnerate the initial ammonia vapor. In the compression system gaseous freon exiting the evaporator is compressed and then condensed in the condenser by heat exchange with the ambient atmosphere. The condensed freon is then returned to the evaporator through a pressure-reducing valve.
German Auslegeschrift No. 1,125,956 discloses a refrigeration system utilizing an absorption system wherein the materials used as refrigerant (e.g., ammonia) and absorbent (e.g., petroleum or paraffin oil) have a miscibility gap in a temperature range below the temperature of the absorber and both are liquid in this range.
In particular, gaseous ammonia is fed, under pressure, into an absorber containing petroleum or paraffin oil under such temperature and pressure conditions as to cause dissolution of the ammonia in the petroleum or paraffin oil. This rich solution is fed through a heat exchanger, where it is cooled, and the miscibility between the ammonia and the petroleum or paraffin oil is reduced to such an extent that separation thereof begins. This partly separated solution is fed into an expeller where the solution is further cooled to the area where there is a pronounced miscibility gap. Consequently, the ammonia and the petroleum or paraffin oil completely separate in the expeller with the lighter liquid ammonia floating on top. The lean petroleum or paraffin oil is withdrawn from the expeller and passed through the heat exchanger where it cools the rich solution. The lean petroleum or paraffin oil is then passed into the absorber. The liquid ammonia is removed from the top of the expeller, passes through an expansion valve and then passes through two series-connected evaporators. In the first evaporator, about 20% of the ammonia evaporates and this evaporator is used to cool the expeller. In the second evaporator, the remaining liquid ammonia evaporates and is used to provide useful refrigeration. Heat of absorption in the absorber is removed by heat exchange with cooling water or air.
Patnode, U.S. Pat. No. 3,990,264, discloses a combination vapor compression-refrigeration cycle wherein the suction end of the compressor is exposed to both vapors discharged from the refrigeration evaporator and a mixture of oil foam and refrigeration vapors discharged from an absorption generator. As the mixture passes through the compressor, it absorbs the heat of compression and is discharged into a heat exchanger where heat energy is transferred to a reclaiming substance. Because of the absorptive process, relatively high temperatures are developed in the compressor discharge whereby the heat energy rejected to the relcaiming substance can be effectively utilized in domestic and industrial heating applications.
Leonard, U.S. Reissue Pat. No. 30,252, discloses a system for high temperature heat recovery in a refrigeration system wherein refrigerant vapors discharged from a compressor are exposed to, and condensed into, a strong absorbent solution to develop temperatures within the mixture that are in excess of the saturation temperature of the discharge vapors. The mixture is brought into a heat exchanger where the high temperature energy is recovered. The diluted absorbent in the mixture is then separated from unabsorbed refrigerant vapors and the dilute absorbent solution is flash cooled by expanding the dilute solution to the inlet pressure of the compressor. The separated unabsorbed refrigerant vapors are indirectly thermally contacted with the flash cooled solution in a concentrator where the unabsorbed refrigerant vapors are condensed, or partially condensed, to boil refrigerant from the dilute solution. The reconcentrated absorbent solution is recycled in the high lift circuit and the freed vapors are delivered to the inlet of the compressor. All of the remaining unabsorbed refrigerant vapors not condensed to concentrate the dilute absorbent solution are passed to a standard refrigeration condenser where they are condensed. The liquid condensate from this refrigeration condenser and the liquid condensate from the concentrator are collected together in a common chamber, the float chamber, and together passed through an expansion device into the evaporator where the liquid refrigerant is again used as the evaporate to accomplish chilling in a conventional manner.
Rojey et al, U.S. Pat. No. 4,420,946, discloses a refrigeration process using a phase separation technique. The technique comprises: compressing a refrigerant fluid and dissolving it in a solvent; cooling the resultant solution to form two distinct phases; separating the liquid phases; recycling the heavy phase; expanding and vaporizing the light phase to produce refrigeration; and recycling the vaporized light phase. A portion of the refrigeration produced is used to cool the aforementioned resultant solution and another portion is used to cool an external medium.
Kaufman, U.S. Pat. No. 4,442,677, discloses a thermal machine having a high-, intermediate-, and low-pressure states, including sealed chambers permitting maintenance of the respective pressures but permitting flow of vapor from one vessel to a second within a stage and permitting flow of an absorbent solution among the vessels in different stages. The intermediate-pressure stage includes resorption and regeneration vessels which are thermally coupled, respectively, to a generation vessel and an absorption vessel in the high- and low-pressure stages, so that a variable fraction of the absorber heat may be transferred to the regenerator and a variable fraction of the resorber heat may be transferred to the generator. This variable internal heat transfer permits the machine to adjust to a wide range of available heat source and heat rejection temperatures while maintaining high efficiency.
Vakil, U.S. Pat. No. 4,179,898, discloses a vapor compression heat pump device having a variable capacity wherein a multi-component working fluid mixture is utilized. The heat pump device comprises a condensing heat exchanger and an associated vapor-liquid separator connected to the compressor, a high-pressure liquid accumulator connected to the condenser and associated separator, a flow restricting device connected to the condenser and associated separator, an evaporating heat exchanger and associated low pressure accumulator connected to the flow restricting device, and the evaporating heat exchanger and low-pressure accumulator connected to the compressor. The capacity of the device is modulated during its heating mode by circulating a multi-component working fluid mixture vapor from the compressor to the condenser. The liquid from the condenser is circulated to the vapor-liquid separator and to the high-pressure accumulator whereby complete condensation is achieved. The mixture is circulated from the separator and the accumulator to the evaporator. The flow of the mixture from the accumulator to the evaporator is controlled selectively in response to changes in the evaporator temperature by the associated flow restricting device. The mixture then flows to a low-pressure accumulator. The density of the vapor in equilibrium with the liquid mixture in the low-pressure accumulator controls the rate of compression or the molar flow of the mixture to and through the compressor.
At higher outdoor temperatures, the complete condensation of and the restricted flow of the working fluid mixture from the vapor-liquid separator and the high-pressure accumulator results in the working fluid mixture which is circulated to the evaporator, being enriched in the high boiling point working fluid component. As the evaporator temperature decreases, the increase of mixture flow from the separator and the high-pressure accumulator enriches the working fluid mixture in the low boiling component. The additional flow of working fluid mixture through the evaporator and to the low-pressure accumulator results in a pressure increase in the low-pressure accumulator. The increase in working fluid mixture in the low-pressure accumulator increases the vapor density. The change from a low to a higher density in the vapor in the low-pressure accumulator increases the flow rate of the mixture through the compressor with a consequent increase in the heat exchanger duties and the compressor power input. Thus, the capacity of the device is modulated in the heating mode.
As may be readily ascertained absorption/desorption systems and vapor compression systems are well known for the transfer of heat, as well as phase separation systems, heat pumps using combined systems and heat pumps using two sources of heat.
Nonetheless, all current heat pump cycles employing non-azeotropic working fluid mixtures have one significant shortcoming: the capacity control is limited to a rather narrow range by the requirement that all liquid in the evaporator has to evaporate completely under steady state operation.
In this regard, conventional heat pumps operating with a single refrigerant as a working fluid show one major disadvantage: with decreasing outdoor temperature the capacity and the coefficient of performance (COP), i.e. the net heat withdrawn from the cold reservoir per unit of work done on the working fluid, decrease very rapidly. Therefore, around freezing temperatures, the heat pump is turned off and other means of heating have to be used. Consequently, heat pumps are under consideration which operate with a non-azeotropic refrigerant mixture. Compared to the conventional heat pump, these new types offer the following advantages: (1) reduced decrease of the capacity with decreasing outdoor temperatures, (2) continuous capacity control within rather narrow limits, and (3) a significant increase in COP, when counterflow heat exchangers can be employed. The first two advantages are achieved by adjusting the composition of the mixture. This can be done by either external control or internal "self-adjustment". The change of composition (at a given temperature) adjusts the pressure in the suction side of the compressor, resulting in a change of the refrigerant mass flow rate and therefor, the system's capacity. The larger the pressure change which can be obtained, the larger the range for capacity adjustments. A large change in pressure can only be achieved when the boiling temperatures of the pure components of the mixture are far apart.
Unlike pure refrigerants, the temperatures of non-azeotropic mixtures change as they evaporate, the size of this temperature change during evaporation being dependent on the difference in the boiling points of the pure components. It is important to note that this difference must not be too large since for given conditions the refrigerant mixture might not be evaporated completely, and could harm the compressor by feeding it a two-phase mixture. This requirement "for complete evaporation" limits the practical application of non-azeotropic refrigerant mixtures in traditional heat pump cycles striving for large capacity adjustments.
Nonetheless, in order to achieve an effective capacity adjustment, a large difference in boiling points (ideally, as large as possible) is desirable, while for the heat pump cycles employed to date only a limited difference in boiling points is acceptable. In order to overcome this dichotomy, a heat pump with solution circuit (HPSC) has been proposed.
In the HPSC, a mixture is chosen where the boiling points of its components are deliberately far apart. The higher boiling component, in fact, is selected so as to not substantially evaporate under the normal operating conditions of the cycle. This higher boiling component instead is recirculated through the heat pump (E. Altenkirch, "Refrigeration Apparatus with Solution Circuit", Kaltetechnik 2 (1950), pp. 251, 279, 310 and G. Alefeld, "Heat Conversion System", to be published).
FIG. 1 illustrates an apparatus utilizing this prior art technique. In particular, a vapor/liquid mixture of the higher boiling and lower boiling components enters the desorber 1 at end 3. In the desorber 1, only the lower boiling component is desorbed and two streams (a vapor rich in the lower boiling component and a liquid rich in the higher boiling component) are formed. The two streams are separated from one another and exit the desorber 1 at end 5. The vapor rich in the lower boiling component is delivered to the compressor 7, compressed therein, and then passed to the absorber 9. The liquid rich in the higher boiling component (absorbent) is supplied to absorber 9 by pump 11 and heat exchanger 23. The vapor is absorbed into the liquid absorbent in absorber 9 and the combined liquid streams leave the absorber and are returned to desorber 1 via pressure-reducing valve 13. The adjustment of composition is easily effected, since the vapor passing through the compressor 7 is almost pure refrigerant (lower boiling component), and the compressed vapor can be controllably rerouted around absorber 9 by activation of control valves 15 and 17. The rerouted vapor can then be condensed and stored in accumulator 19. The stored condensed vapor can then be controllably fed to the desorber 1 via pressure-reducing valve 21. The cycle is a closed cycle.
In order to provide a more intuitive understanding of the operation of this heat pump cycle and the apparatus of FIG. 1, FIG. 2 shows a log (pressure) vs--1/T diagram for the cycle with vapor pressure lines for the lower boiling component (refrigerant) and the higher boiling component (absorbent) indicated. Superimposed on the graph, are elements of the apparatus so that pressure, temperature and composition changes within those heat exchangers which accommodate a phase change become obvious from the graph. Dashed lines d.sub.i (i=1, 2 or 3) indicate the direction of the change of composition with decreasing outdoor temperature (d.sub.3 representing the direction of change at a higher temperature than d.sub.2 which in turn represents the direction of change at a higher temperature than d.sub.1). It thus becomes apparent that with decreasing outdoor temperature, the suction side pressure (and therefore the capacity) can be increased by mere adjustment of the composition of the incoming liquid stream. (The capacity can also be adjusted at constant outdoor temperature to meet a varying load.)
There is another advantage to this design which is not obtainable from conventional heat pumps. The circulating solution allows an efficient internal heat exchange, so that flashing at the desorber inlet is considerably reduced, thus increasing the capacity without changing the mass flow rate. This internal heat exchange requires an additional heat exchanger which is indicated in FIG. 1, as the element 23 (a countercurrent heat exchanger).
An obvious disadvantage of this heat pump cycle, i.e. the HPSC, is the fact that a solution pump is necessary. The additional heat exchanger also adds to the cost of the unit but, on the other hand, this expedient has been considered for conventional heat pumps utilizing mixed fluids, since it can increase capacity. However, there is still one problem inherent in all of the heat pump cycles discussed: with decreasing outdoor temperature the pressure ratio will increase in order to maintain the absorber (condenser) at the required high indoor temperature.